The present invention relates to processes for the preparation of chalcogenide alloys and to processes for suppressing the fractionation of chalcogenide alloys. More specifically, the present invention relates to processes for controlling the fractionation of chalcogenide alloys during the vacuum deposition thereof. In one embodiment, the present invention is directed to a process for controlling and suppressing the fractionation of chalcogenide alloys which comprises adding to the source material selected, such as the source alloy trigonal selenium thereby enabling, for example, desirable chalcogenide alloys possessing homogeneous distributions of the chalcogens throughout the product obtained. The products resulting from the process of the present invention can be selected as photoconductors in electrophotographic imaging systems, including xerographic imaging and printing methods.
Chalcogens and chalcogenide alloys, and their use in electrophotographic processes, are well known. Generally, the aforementioned photoconductors are prepared by known vacuum deposition, flash evaporation, and chemical vapor deposition methods. These methods possess disadvantages in some instances, thus for example with vacuum deposited chalcogenide alloys the products obtained usually lack controllable reproducibility in their homogeneity thereby adversely effecting the electrophotographic electrical characteristics thereof. As the elements selected in the source alloy for vacuum deposition usually have different vapor pressures, such elements tend to separate during the vacuum deposition process causing undesirable inhomogeneity, or fractionation thereof. Also, with vacuum deposition processes the components with high selenium content tend to evaporate first and thus are not subsequently available for deposition. Accordingly, the final photoreceptor will contain less selenium at the top surface thereof which adversely affects the electrical characteristics thereof, and adds significantly to the cost of photoreceptor manufacturing or preparation primarily as a result of a reduction in total yield, or yield loss. With the processes of the present invention these and other disadvantages are avoided.
Electrophotographic photoconductive imaging members containing amorphous selenium can be modified to improve panchromatic response, increase speed, and to improve color copyability. These members are usually comprised of binary and ternary chalcogenide alloys such as alloys of selenium with tellurium and/or arsenic with and without halogens. The selenium imaging members may be fabricated as single layered devices comprising a selenium-tellurium, selenium-arsenic, selenium antimony, selenium tellurium arsenic, selenium tellurium bismuth, selenium tellurium arsenic chlorine, selenium-tellurium-antimony or selenium-tellurium-arsenic alloy layer, which functions as a charge generation and charge transport medium. The selenium electrophotographic imaging members may also comprise multiple layers such as, for example, a selenium alloy transport layer and a selenium alloy generator layer.
One known process for the preparation of photoconductors comprises the vacuum deposition of a selenium alloy on a supporting substrate such as aluminum. Tellurium can then be incorporated therein as an additive primarily for the purpose of enhancing the spectral sensitivity thereof. Also, arsenic can be incorporated as an additive for the primary purpose of improving wear characteristics, passivation against crystallization, and improving electrical performance of the resulting photoconductor or photoreceptor. Generally, the tellurium addition can be incorporated as a thin selenium-tellurium alloy layer deposited over a selenium alloy layer to achieve the benefits of the photogeneration and transport characteristics. Fractionation of chalcogenide alloys during the vacuum evaporation processes results in an undesirable concentration gradient of, for example, tellurium and/or arsenic in the deposited photoconductor. Accordingly, there results inhomogeneities (fractionation) in the stoichiometry of the vacuum deposited thin films. Fractionation occurs as a result of differences in the partial vapor pressure of the molecular species of the solid and liquid phases of binary, ternary and other multicomponent alloys. An important aspect in the fabrication of chalcogenide based photoreceptors resides in controlling the fractionation of alloy components such as tellurium and/or arsenic during the evaporation of source alloys. More specifically, tellurium and/or arsenic fractionation control is particularly important since the tellurium and/or arsenic concentration at the top surface of the resulting photoreceptor affects xerographic sensitivity, charge acceptance, dark discharge, copy quality, photoreceptor wear, cost of fabrication, crystallization resistance, and the like. For example, in single layer low arsenic selenium alloy photoreceptors arsenic enrichment at the top surface caused by fractionation can also cause severe reticulation of the evaporated film. Further, in single layer tellurium selenium alloy photoreceptors, tellurium enrichment at the top surface due to fractionation can cause undue sensitivity enhancement, poor charge acceptance and enhancement of dark discharge. Also, in two layer or multilayer photoreceptors where low arsenic alloys may be incorporated as a transport layer, arsenic enrichment at the interface with the layer above can lead to residual cycle up problems. Moreover, in two layer or multilayer photoreceptors where tellurium alloys may be incorporated as a generator layer, tellurium enrichment at the upper surface of the charge generator layer can result in similar undue sensitivity enhancement, poor charge acceptance, and enhancement of dark discharge.
One specific method of preparing selenium alloys for evaporation comprises the grinding of selenium alloy shots (beads) and compressing the ground material into pellet agglomerates typically from about 150 to about 300 milligrams in weight and having an average diameter of about 6 millimeters (6,000 micrometers). The pellets are then evaporated from crucibles in a vacuum coater in a manner designed to minimize the fractation of the alloy during evaporation. One disadvantage of the aforementioned vacuum deposited photoconductors, such as selenium-tellurium alloy layer, is the crystallization of the selenium-tellurium alloy at the surface of the layer when exposed to heat. To retard premature crystallization and extend photoreceptor life, the addition of up to about 5 percent arsenic to the selenium-tellurium alloy can be beneficial without impairment of xerographic performance. For example, when the photoreceptor comprises a single layer selenium arsenic alloy, about 1 to about 2.5 percent by weight arsenic, based on the weight of the entire layer, at the surface of the alloy layer there is provided protection against surface crystallization. When the concentration of arsenic is greater than about 2.5 percent by weight, electrical instability risks increase.
Also, in deposited layers of selenium tellurium alloys the amounts of top surface tellurium present can cause excessively high photosensitivity. This photosensitivity is variable and changes as the surface of the layer wears away. Surface injection of corona deposited charge and thermally enhanced bulk dark decay involving carrier generation cause the toner images in the final copies to exhibit a washed out, low density appearance. Excessive dark decay causes loss of high density in solid areas of toner images and general loss of image density.
One known method for attempting to control fractionation is the selection of shutters for incorporation over the evaporation crucibles. Shutters are normally selected after evaporation is substantially completed to avoid the coating of, for example, tellurium and arsenic rich species on the photoreceptor. This results in a photoreceptor or photoconductor with a top surface containing desired levels of tellurium and arsenic. Further, shutters can be utilized at inititation of evaporation of elements from the crucible to avoid sputtering. The aforesaid shuttering is generally costly, usually requires incomplete evaporation, and further the crucibles selected have to be cleaned after each evaporation. Furthermore, with shuttering generally a substantial amount of the source material is lost during the process.
Accordingly, a problem encountered in the fabrication of chalcogenide alloy photoreceptors, such as selenium alloy photoreceptors, is the fractionation or preferential evaporation of a component whereby the resulting film composition is not equivalent to the source component, such as the source alloy, thus the deposited film or layer does not have a uniform composition extending from one surface to the other. For example, with selenium tellurium alloys containing from about 10 to about 60 percent by weight of tellurium, the tellurium concentration is high at the top surface and very low, that is it approaches almost zero, at the bottom of the vacuum deposited layer. This problem is observed with, for example, alloys of Se-Te, Se-As, Se-As-Te, Se-As-Te-Cl, mixtures thereof, and the like.
In copending application U.S. Pat. No. 4,770,965, there is disclosed a process which includes heating an alloy comprising selenium and from about 0.05 percent to about 2 percent by weight arsenic until from about 2 percent to about 90 percent by weight of the selenium in the alloy is crystallized, vacuum depositing the alloy on a substrate to form a vitreous photoconductive insulating layer having a thickness of between about 100 micrometers and about 400 micrometers containing between about 0.3 percent and about 2 percent by weight arsenic at the surface of the photoconductive insulating layer facing away from the conductive substrate, and heating the photoconductive insulating layer until only the selenium in the layer adjacent the substrate crystallizes to form a continuous substantially uniform crystalline layer having a thickness up to about one micrometer. A thin protective overcoating layer is applied on the photoconductive insulating layer. The selenium-arsenic alloy may be partially crystallized by placing the selenium alloy in shot form in a crucible in a vacuum coater and heating to between about 93.degree. C. (200.degree. F.) and about 177.degree. C. (350.degree. F.) for between about 20 minutes and about one hour to increase crystallinity and avoid reticulation. Preferably, the selenium-arsenic alloy material in shot form is heated until from about 2 percent to about 90 percent by weight of the selenium in the alloy is crystallized. The selenium-arsenic alloy material shot may be crystallized completely prior to vacuum deposition to ensure that a uniform starting point is employed. However, if desired, a completely amorphous alloy may be used as the starting material for vacuum deposition. In Examples II and V of this copending patent application, halogen doped selenium-arsenic alloy shot contained about 0.35 percent by weight arsenic, about 11.5 parts per million by weight chlorine, and the remainder selenium, based on the total weight of the alloy was heat aged at 121.degree. C. (250.degree. F.) for 1 hour in crucibles in a vacuum coater to crystallize the selenium in the alloy. After crystallization, the selenium alloy was evaporated from chrome coated stainless steel crucibles at an evaporation temperature of between about 204.degree. C. (400.degree. F.) and about 288.degree. C. (550.degree. F.).
Copending application U.S. Pat. No. 4,780,386, discloses a process wherein the surfaces of large particles of an alloy comprising selenium, tellurium and arsenic, the particles having an average particle size of at least 300 micrometers and an average weight of less than about 1,000 milligrams, are mechanically abraded while maintaining the substantial surface integrity of the large particles to form between about 3 percent by weight to about 20 percent by weight dust particles of the alloy based on the total weight of the alloy prior to mechanical abrasion. The alloy dust particles are substantially uniformly compacted around the outer periphery of the large particles of the alloy. The large particles of the alloy may be beads of the alloy having an average particle size of between about 300 micrometers and about 3,000 micrometers or pellets having an average weight between about 50 milligrams and about 1,000 milligrams, the pellets comprising compressed finely ground particles of the alloy having an average particle size of less than about 200 micrometers prior to compression. In one preferred embodiment, the process comprises mechanically abrading the surfaces of beads of an alloy comprising selenium, tellurium and arsenic having an average particle size of between about 300 micrometers and about 3,000 micrometers while maintaining the substantial surface integrity of the beads to form a minor amount of dust particles of the alloy, grinding the beads and the dust particles to form finely ground particles of the alloy, and compressing the ground particles into pellets having an average weight between about 50 milligrams and about 1,000 milligrams. In another embodiment of the copending application, mechanical abrasion of the surface of the pellets after the pelletizing step may be substituted for mechanical abrasion of the beads. The process includes providing beads of an alloy comprising selenium, tellurium and arsenic having an average particle size of between about 300 micrometers and about 3,000 micrometers, grinding the beads to form finely ground particles of the alloy having an average particle size of less than about 200 micrometers, compressing the ground particles into pellets having an average weight between about 50 milligrams and about 1,000 milligrams, and mechanically abrading the surface of the pellets to form alloy dust particles while maintaining the substantial surface integrity of the pellets.
In copending application U.S. Pat. No. 4,822,712, the disclosure of which is totally incorporated herein by reference, there are illustrated processes for controlling fractionation. More specifically, there are disclosed in this copending application processes for crystallizing particles of an alloy of selenium comprising providing particles of an alloy comprising amorphous selenium and an alloying component selected from the group consisting of tellurium, arsenic, and mixtures thereof, said particles having an average size of at least about 300 micrometers and an average weight of less than about 1,000 milligrams, forming crystal nucleation sites on at least the surface of said particles while maintaining the substantial integrity of said particles, heating the particles to at least a first temperature between about 50.degree. C. and about 80.degree. C. for at least about 30 minutes to form a thin, substantially continuous layer of crystalline material at the surface of the particles while maintaining the core of selenium alloy in said particles in an amorphous state, and rapidly heating said particles to at least a second temperature below the softening temperature of said particles, the second temperature being at least 20.degree. C. higher than the first temperature and between about 85.degree. C. and about 130.degree. C. to crystallize at least about 5 percent by weight of said amorphous core of selenium alloy in the particles.
In application U.S. Ser. No. 270,184 entitled Processes for Preparing and Controlling the Fractionation of Chalcogenide Alloys with the listed inventors of Geoffrey M. T. Foley, Paul Cherin, and Santokh S. Badesha, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for the preparation of chalcogenide alloys which comprises providing a chalcogenide alloy source component; crystallizing the source component; and evaporating the source component in the presence of an organic component.
Also, there is described in U.S. Pat. No. 4,205,098 a process wherein a powdery material of selenium alone or at least with one additive is compacted under pressure to produce tablets, the tablets being degassed by heating the tablets at an elevated temperature below the melting point of the metallic selenium, and thereafter using the tablets as a source for vacuum deposition. The tablets formed by compacting the powdery selenium under pressure may be sintered at a temperature between about 100.degree. C. and about 220.degree. C. Typical examples of sintering conditions include 210.degree. C. for between about 20 minutes and about 1 hour and about 1 to about 4 hours at 100.degree. C. depending upon compression pressure. Additives mentioned include Te, As, Sb, Bi, Fe, Tl, S, I, F, Cl, Br, B, Ge, PbSe, CuO, Cd, Pb, BiCl.sub.3, SbS.sub.3, Bi.sub.2, S.sub.3, Zn, CdS, CdSe, CdSeS, and the like.
With further respect to the prior art, there is mentioned U.S. Pat. Nos. 4,609,605, which illustrates a multilayered electrophotographic imaging member wherein one of the layers may comprise a selenium-tellurium-arsenic alloy prepared by grinding selenium-tellurium-arsenic alloy beads, with or without halogen doping, preparing pellets having an average diameter of about 6 millimeters from the ground material, and evaporating the pellets in crucibles in a vacuum coater; 4,297,424, which describes a process for preparing a photoreceptor wherein selenium-tellurium-arsenic alloy shot is ground, formed into pellets and vacuum evaporated; 4,554,230, which discloses a method for fabricating a photoreceptor wherein selenium-arsenic alloy beads are ground, formed into pellets and vacuum evaporated; 3,524,754 directed to a process for preparing a photoreceptor wherein selenium-arsenic-antimony alloys are ground into fine particles and vacuum evaporated; and 4,710,442 relating to an arsenic-selenium photoreceptor, wherein the concentration of arsenic increases from the bottom surface to the top surface of the photoreceptor, that the arsenic concentration is about 5 weight percent at a depth about 5 to 10 microns on the top surface of the photoreceptor and is about 30 to 40 weight percent at the top surface of the photoreceptor, which photoreceptor can be prepared by heating a mixture of selenium-arsenic alloys in a vacuum in a step-wise manner such that the alloys are consequentially deposited on the substrate to form a photoconductive film with an increasing concentration of arsenic from the substrate interface to the top surface of the photoreceptor. In one specific embodiment, a mixture of 3 selenium-arsenic alloys are maintained at an intermediate temperature in the range of from about 100.degree. to 130.degree. C. for a period of time sufficient to dry the mixture. Further, in U.S. Pat. No. 4,583,608 there is disclosed the heat treatment of single crystal super alloy particles by using a heat treatment cycle during the initial stages of which incipient melting occurs within the particles being treated. During a subsequent step in the heat treatment process, substantial diffusion occurs in the particle. In a related embodiment, single crystal articles which have previously undergone incipient melting during a heat treatment process are prepared by a heat treatment process. In still another embodiment, a single crystal composition of various elements including chromium and nickel is treated to heating steps at various temperatures. Other prior art includes U.S. Pat. Nos. 4,585,621; 4,632,849; 4,484,945; 4,414,179; 4,015,029, and 3,785,806; Swiss Patent CH 656486 A5; and Japanese Pat. Nos. 60-172346 and 57-91567.
There is illustrated in U.S. Pat. No. 4,513,031 a process for the formation of an alloy layer on the surface of a substrate, which for example comprises forming in a vessel a molten bath comprising at least one vaporizable alloy component having a higher vapor pressure than at least one other vaporizable alloy component in the bath, forming a thin substantially inert liquid layer of an evaporation retarding film on the upper surface of the molten bath, the liquid layer of the evaporation retarding film having a lower or comparable vapor pressure than both the vaporizable alloying component having a higher vapor pressure and the other vaporizable alloying component, covaporizing at least a portion of both the vaporizable alloying component having a higher vapor pressure and the other vaporizable alloying component whereby the evaporation retarding film retards the initial evaporation of the vaporizable alloying component having a higher vapor pressure, and forming an alloy layer comprising both the vaporizable alloying component having a higher vapor pressure and the other vaporizable alloying component on the substrate, see column 3, lines 33 to 54, for example. Examples of vaporizable alloying components include selenium-sulfur and the like, and examples of vaporizable alloying components having relatively low vapor pressures which include tellurium, arsenic, antimony, bismuth, and the like are illustrated in column 4, reference for example lines 41 to 50. Examples of suitable evaporation retarding film materials are outlined in column 4 at line 54, and continuing onto column 5, line 36, such materials including inert oils, greases or waxes at room temperature which readily flow less than the temperature of detectable deposition of the vaporizable alloying components having higher vapor pressures in the alloying mixture, and may include, for example, long chain hydrocarbon oils, greases, and waxes, lanolin, silicone oils such as dimethylpolysiloxane, branched or linear polyolefins such as polypropylene wax and polyalpha olefin oils, and the like, see column 5. According to the teachings of this patent, optimum results are achieved with high molecular weight long chain hydrocarbon oils and greases generally refined by molecular distillation to have low vapor pressure at the alloy deposition temperature, see column 5, lines 32 to 36. It is believed with the aforementioned process that the levels of organics, which are incorporated into the resulting alloy film, are sufficiently high causing negative adverse effects in the electrical properties of the resulting photoreceptor, for example, dark decay and cyclic stability are adversely effected.